A radar device radiates a radio wave from a measurement point into a space and receives a signal of a reflected wave reflected from a target, to measure the distance, direction or the like from the measurement point to the target. Particularly, in recent years, a radar device has been developed that is capable of detecting a pedestrian or the like as well as an automobile as a target by performing high resolution measurement using a radio wave of a short wavelength such as a micro wave or a millimeter wave.
Further, the radar device receives a mixed signal of a reflected wave from a target in a short distance and a reflected wave from a target in a long distance. In particular, in a case where a range sidelobe occurs by the signal of the reflected wave from the target in a short distance, the range sidelobe and a main lobe of the signal of the reflected wave from the target in a long distance may be mixedly present. In this case, detection accuracy when the radar device detects the target in a long distance may be deteriorated.
Further, in a case where an automobile and a pedestrian are present in the same distance from the measurement point, the radar device may receive a mixed signal of signals of respective reflected waves from the automobile and the pedestrian having different radar cross sections (RCS). In general, it is said that the radar cross section of the pedestrian is lower than the radar cross section of the automobile. Thus, for example, even though the automobile and the pedestrian are present in the same distance from the measurement point, it is necessary for the radar device to appropriately receive the reflected wave from the pedestrian as well as the automobile.
Thus, in the radar device in which high resolution measurement is necessary with respect to a plurality of targets, it is necessary to perform transmission of a pulse wave or a pulse modulated wave having an auto-correlation characteristic of a low range sidelobe level (hereinafter, referred to as a “low range sidelobe characteristics”). Further, in the radar device, it is necessary to secure such a wide reception dynamic range as to receive signals of reflected waves of various reception levels according to the distance or type of the target.
With regard to the pulse wave or the pulse modulated wave having the above-mentioned low range sidelobe characteristics, a pulse compression radar that transmits a high frequency transmission signal using complementary codes has been proposed in the related art. Here, the pulse compression refers to a technique in which the radar pulse-modulates or phase-modulates a pulse signal and transmits the result using a signal of a wide pulse width, and demodulates (compresses) a received signal in signal processing after reception of a reflected wave and converts the result into a signal of a narrow pulse width, to thereby equivalently increase reception power. According to the pulse compression, it is possible to increase a detection distance of the target, and to enhance distance estimation accuracy for the detection distance.
The complementary codes is formed using a plurality of, for example, two complementary code sequences (an, bn). Further, the complementary codes has a characteristic that, in respective auto-correlation calculation results of one complementary code sequence an and the other complementary code sequence bn, by causing delay times τ (second) to match with each other and adding the respective auto-correlation calculation results, the range sidelobe becomes zero. Here, a parameter n is 1, 2, . . . , L. A parameter L represents a code sequence length, or simply a code length.
A method of generating complementary codes is disclosed in NPL 1, for example. Here, a simple method of generating complementary codes will be described with reference to FIG. 12. FIG. 12 is a diagram illustrating an example of a general generation procedure of a code sequence of complementary codes. As shown in FIG. 12, sub code sequences (c, d) having a code length L=2Z-1 formed using an element 1 or an element −1 are generated from disclosures in the fourth row and the fifth row, and code length complementary code sequences (a, b) having a code length L=2Z are generated from disclosures in the sixth row and the seventh row.
Here, one complementary code sequence a is obtained by connecting the sub code sequence c and the sub code sequence d. The other complementary code sequence b is obtained by connecting the sub code sequence c and a sub code sequence −d.
In FIG. 12, the code sequences a and b respectively represent complementary code sequences, and the code sequences c and d respectively represent sub code sequences that form the complementary code sequences. Further, a parameter Z defines a code length L of the respective generated complementary code sequences (a, b).
The characteristic of such complementary codes will be described with reference to FIG. 13. FIG. 13 is a diagram illustrating characteristics of complementary codes. (a) in FIG. 13 is a diagram illustrating an auto-correlation value calculation result of one complementary code sequence an. (b) in FIG. 13 is a diagram illustrating an auto-correlation value calculation result of the other complementary code sequence bn. (c) in FIG. 13 is a diagram illustrating a value obtained by adding the auto-correlation value calculation results of two complementary code sequences (an, bn). The code length L of the complementary codes used in FIG. 13 is 128.
The auto-correlation value calculation result of one complementary code sequence an among the complementary code sequences (an, bn) is calculated according to Formula (1). The auto-correlation value calculation result of the other complementary code sequence bn among the complementary code sequences (an, bn) is calculated according to Formula (2). A parameter R represents an auto-correlation value calculation result. Here, in a case where n>L or n<1, the complementary code sequences an and bn, become zero (that is, in n>L or n<1, an=0 and bn=0). Asterisk * represents a complex conjugate operator.
                    [                  Exp          .                                          ⁢          1                ]                                                                                  R            aa                    ⁡                      (            τ            )                          =                              ∑                          n              =              1                        L                    ⁢                                    a              n                        ⁢                          a                              n                +                τ                            *                                                          (        1        )                                [                  Exp          .                                          ⁢          2                ]                                                                                  R            bb                    ⁡                      (            τ            )                          =                              ∑                          n              =              1                        L                    ⁢                                    b              n                        ⁢                          b                              n                +                τ                            *                                                          (        2        )            
As shown in FIG. 13(a), in an auto-correlation value calculation result Raa(τ) of the complementary code sequence an calculated according to Formula (1), a peak occurs when a delay time (or shift time) τ is zero, and a range sidelobe is present when the delay time τ is not zero. Similarly, as shown in FIG. 13(b), in an auto-correlation value calculation result Rbb(τ) of the complementary code sequence b calculated according to Formula (2), a peak occurs when the delay time τ is zero, and a range sidelobe is present when the delay time τ is not zero.
As shown in FIG. 13(c), in an added value of the auto-correlation value calculation results (Raa(τ), Rbb(τ)), a peak occurs when the delay time τ is zero, and a range sidelobe is not present and zero is obtained when the delay time τ is not zero. Hereinafter, the peak occurring when the delay time τ is zero is referred to as a “main lobe”. This relationship is expressed by Formula (3). In (a) to (c) of FIG. 13, the transverse axis represents a delay time (τ) in auto-correlation value calculation, and the longitudinal axis represents a calculated auto-correlation value calculation result.[Exp. 3]Raa(τ)+Rbb(τ)≠0,when τ=0Raa(τ)+Rbb(τ)=0,when τ≠0  (3)
In a case where a mixed signal of reflected waves from a target in a short distance and a target in a long distance is received, in general, it is known that as the code length of the codes passed through pulse compression is increased, a necessary reception dynamic range is increased.
However, using the above-mentioned complementary codes, it is possible to decrease a peak sidelobe in a shorter code length. Thus, in the complementary codes using a short code length, in a case where the mixed signal of the reflected waves from the target in a short distance and the target in a long distance is received, it is also possible to decrease a reception dynamic range.
Further, as an example of the above-mentioned radar device, a configuration has been disclosed in which when a target is detected, a plurality of radars is provided that respectively measures individual measurement areas. In the related art, a wide area radar device has been proposed that individually controls the plurality of radars and detects the target in each measurement area.
Hereinafter, each radar that respectively measures the individual measurement area when the target is detected is referred to as a “sector radar”. The respective measurement areas of the respective sector radars are individually separated, but may be partially overlapped with each other in a case where the measurement areas are close to each other.
As mentioned above, in the wide area radar device in the related art, in a case where the measurement areas of the respective sector radars are close to each other, interference occurs between the transmission signals transmitted from the respective sector radars. In a case where the interference occurs, the wide area radar device in the related art causes a problem that position measurement estimation accuracy of the target is deteriorated.
With regard to the above problem, in order to reduce the occurrence of interference between the sector radars in the wide area radar device in the related art, the following methods have been studied.
A first method is to divide a frequency band used by each sector radar into a plurality of different frequency bands or frequency bands (sub-bands) of a predetermined narrow band and to perform FDM (frequency division multiplexing) for a transmission signal to transmit the transmission signal.
According to the first method, it is possible to suppress the occurrence of interference between the respective sector radars by using the different frequency bands, but the following problem arises. That is, in the former case where the plurality of different frequency bands is used, there is a problem that a large amount of frequency resources are necessary. Further, in the latter case where the frequency bands of the narrow band are used, there is a problem that time resolution of position measurement estimation (corresponding to distance resolution) of a target in each sector radar is reduced.
A second method is to perform CDM (code division multiplexing) for a transmission signal using a plurality of code sequences having low cross-correlation to transmit the transmission signal, in each sector radar. According to the second method, addition of new frequency bands and sub-bands is not necessary, and thus, time resolution of position measurement estimation of a target in each sector radar is not reduced.
However, in a case where the transmission signal passes through CDM for each sector radar for transmission, the transmission signal is asynchronously received from a different sector radar, and as a result, interference between codes occurs between the respective sector radars. Further, in general, an auto-correlation characteristic of a code sequence having low cross-correlation is not superior, and as a result, a range sidelobe becomes large.
Thus, in the radar device in the related art, there is a problem that detection performance is deteriorated in a case where a mixed signal of a plurality of reflected waves from a target present in a short distance and a target present in a long distance is separated to detect each target.
A third method is to use a perfect complementary sequence system disclosed in PTL 1, in which low range sidelobe characteristics of complementary codes is satisfied and interference between codes occurring between respective sector radars is reduced.
Two radar systems A and B disclosed in PTL 1 perform transmission and reception, using different coded pulses that are P1 and P2 in the radar system A and Q1 and Q2 in the radar system B as coded pulses of perfect complementary sequences, and using a carrier wave of the same frequency band.
In this case, in a case where a plurality of coded pulses transmitted from a host radar system is received, a plurality of auto-correlation function signals RP1P1(τ) and RP2P2(τ) or RQ1Q1(τ) and RQ2Q2(τ) respectively corresponding to the plurality of coded pulses P1 and P2 or Q1 and Q2 is output. On the other hand, in a case where the host radar system receives a plurality of coded pulses transmitted from a different radar system, a plurality of cross-correlation function signals RQ1P1(τ) and RQ2P2(τ) or PP1Q1(τ) and RP2Q2(τ) respectively corresponding to the plurality of coded pulses transmitted from the different radar system is output.
From a characteristic of the perfect complementary sequences, the sum (RP1P1(τ)+RP2P2(τ) or RQ1Q1(τ)+RQ2Q2(τ)) of a plurality of outputs of auto-correlation function signals is 0 when τ is not 0, and the sum (RQ1P1(τ)+RQ2P2(τ) or PP101(τ)+RP2Q2(τ)) of a plurality of outputs of cross-correlation function signals is 0 regardless of τ. Thus, with respect to the plurality of coded pulses (P1 and P2 or Q1 and Q2) transmitted from the host radar system, the reception side performs a reception process of calculating the plurality of corresponding auto-correlation function signals to thereby obtain compressed pulses with no sidelobe. Similarly, in a case where the plurality of coded pulses transmitted from the different radar system is received, signal components of the different radar system may be set to 0 in a process of calculating the sum of the auto-correlation function signals. Thus, it is possible to provide a plurality of radar systems with no mutual interference even using the same frequency band in adjacent frequency bands.